1. Field of the Invention
The present invention relates, in general, to control systems, and, more particularly, to circuits, systems and methods for providing automatically gain control with self-adaptive attack and decay times.
2. Relevant Background
Automatic gain control (AGC) circuits are common components in a wide variety of analog and digital systems. For example, when reading signals from a disk surface the signal amplitude produced by the read head may vary significantly and benefits from automatic gain control to scale the signal magnitude before further signal processing. In communication systems, receivers, tuners and demodulators often require AGC processing of received signals to account for variations in the receive channel. AGC circuits are also used to prevent saturation in analog-to-digital converters. Other applications for AGC circuits are known. AGC circuit attempt to maintain relatively constant output signal amplitude over a range of signal input variations. This is typically achieved with an AGC which averages the output signal from the receiver and generates a feedback signal, referred to herein as an AGC control signal. The AGC control signal is coupled to control the gain of a variable gain amplifier.
AGC control systems have several characteristics which limit their use in a variety of applications. For example, AGC systems have a characteristic delay in its response to changes in the magnitude of the input signal. This means that the AGC control voltage remains constant for a short time after a change in the input signal level, after which the AGC control voltage follows the change to compensate for the level change. This delay is referred to as the “attack time” when describing the AGC system response to an input signal of increasing magnitude, and is referred to as a “decay time” when describing the AGC system response to an input signal of decreasing magnitude. The conventional AGC technology exhibits different or asymmetric attack and decay times. Normally, fast attack and slow decay are present.
FIG. 1 illustrates a conventional AGC circuit 100 consistent with practice in the prior art. In FIG. 1, a differential input identified as VINP and VINN is applied to the input of variable gain amplifier (VGA) 101. VGA 101 produces an amplified output (VOUT) where the magnitude of the amplification is determined by the magnitude of a signal present on a control node of VGA 101. As shown in FIG. 1, conventional AGC circuits generate an AGC control voltage (VAGC) by charging a capacitor 111 in a resistor-capacitor (RC) circuit. The AGC control voltage is coupled to a control node of a variable gain amplifier 101. In operation, when the output voltage is larger than a pre-determined reference level, level detector 105 is triggered and closes switch 107 for a specified duration. While switch 107 is closed, a constant current provided by currents source 113 charges capacitor 111. Usually capacitor 111 is implemented as an external capacitor because it requires a relatively large capacitance that is not practical to implement in an integrated fashion. The voltage on capacitor 111 is coupled to the control node of VGA 101 through buffer 109.
The attack time is determined by the rate at which the voltage on capacitor 111 can be increased. The increase step voltage on the capacitor in every charge cycle is described by:
      Δ    ⁢                  ⁢    V    =                    (                  I          ·          t                )            C        =          I              (        fC        )            where f is the signal frequency of VINP and VINN. This equation illustrates that the attack time has a direct dependence on the signal frequency. In order to obtain an acceptable attack time, a large capacitance, which must typically be implemented externally, is required. Further, the lower the input signal frequency, the large the capacitor that is required.
It can also be seen in FIG. 1 that the voltage on capacitor 111 is only driven in one direction or sense. A resistor is provided to gradually and continuously drain current away and discharge capacitor 111. While this has the advantage of simplicity, the rate of current flow through the resistor changes continuously depending on the voltage across the capacitor. Further, while current is being applied through switch 107, a portion of the current is being drained off by the resistor. As a result, VAGC changes in a non-linear fashion that makes precise control more difficult.
In view of the above it is apparent that there is a need for improved systems, methods and circuits for automatic gain control.